It is common to equip construction vehicles, such as wheel loaders, with automatic transmission controls which automatically shift the transmission in response to sensed operating parameters such as vehicle speed. Such controls typically include a gear selector for selecting a desired gear ratio and direction of travel. Gear changes are automatically effected up to the desired gear ratio in accordance with a shift map which relates the sensed parameter to gear ratio. Such controls are advantageous because the operator is not required to manually shift gear ratios and, hence, his/her hands are free to operate other controls such as implement and steering controls.
Wheel loaders are commonly used to load raw material from a stock pile into a dump truck as illustrated in FIG. 1. Initially, the loader 1 approaches the pile 2 in a forward gear such as second gear forward FWD2 (operation 1). As the loader approaches the pile, a digging implement (bucket) is positioned to engage the pile. As the implement enters the pile, the vehicle's momentum is slowed and the transmission automatically shifts into first gear forward FWD1 (operation 2) as vehicle speed drops below the programmed shiftpoint. At the same time, the bucket is manipulated (i.e., raised) to obtain a load of material. The transmission is then manually shifted into a reverse gear using the gear selector, and the vehicle backs away from the stock pile (operation 3). Subsequently, the direction of the loader is changed so that the loader approaches the dump truck 3 (operation 4). Typically, the loader approaches the truck at a high speed in second gear forward F2. When the bucket is positioned over the truck, the load is dumped into the truck. The loader then backs away from the truck in a reverse gear ratio (operation 5). The above-described work cycle is repeated until the desired amount of material is deposited onto the truck.
Conventional automatic transmission controls are disadvantageous because entering the pile in second gear lugs down engine speed, thereby reducing the power available to operate the implement hydraulics. In order to avoid this problem it is desirable to effect a downshift to first gear forward just prior to entering the pile, thereby sustaining vehicle momentum and maintaining a high engine speed as the implement enters the pile. Similarly, it is desirable to downshift to first as the loader approaches the truck in order to maintain a high engine speed for rapidly lifting the bucket to its dump height. These are complicated operations in a conventional system because the operator is required to use one hand to control a steering device and one hand to manipulate an implement control lever. However, when a downshift is required, the operator must use one of his hands to manually effect the gear change.
In response to the above problems, U.S. Pat. No. 4,800,660, hereinafter '660, which issued on Jan. 31, 1989, provides a system which automatically downshifts the transmission when the implement is moved to a position suitable for digging. More specifically, the transmission is shifted into a lower gear ratio when the boom and bucket assume a preselected angle with respect to the loader. This system is problematic from the standpoint that the downshift occurs as soon as the implement assumes the preselected position. As such, it is difficult to precisely and correctly time the occurrence of the downshift. As was mentioned above, it is also desirable to be able to affect downshifts as the loader approaches the dump truck. '660 makes no mention of adapting its control to enable this type of downshift, nor would the disclosed system be readily adaptable to enable such downshifts. This is because a raised bucket position is used when approaching the truck and also when loading the vehicle. However, the '660 system provides no means for differentiating when the bucket is raised for loading and when it is raised for dumping.
Systems are also known which provide a downshift switch on an implement control lever. Such systems are advantageous because the desired downshifts can be effected by the same hand used to operate the implement control lever. However, known systems are disadvantageous because they permit excessive vehicle jerk and engine braking during a forced downshift. This is because downshifts are typically permitted at any ground speed, regardless of engine speed. If engine speed is low and ground speed is high, the transmission input speed following the downshift can greatly exceed the engine speed. In this situation excessive jerk and engine braking will occur.
One known system which provides a downshift switch is described in Japanese Patent Laid-open Number Sho 63-275432 titled "Automatic Speed Change Control Device of Shovel Loader." This system only permits automatic shifting when the vehicle speed is at full throttle. When the vehicle is not at full throttle, it appears that gear changes must be effected manually. The downshift switch is only operative when automatic shifting is operative, i.e. at full throttle. This system is disadvantageous because it only permits operation of the downshift switch at full-throttle and during automatic shifting. In many instances, it is desirable to allow downshifting using the downshift switch in the manual mode or when the engine speed is not at full throttle.
The present invention is directed towards overcoming one or more of the problems set forth above.